qdoc: skip the system include hack on MS-Win

This is a fix-up for "qdoc: Find the system include path if not
provided"; MSVC complained about "definition of dllimport function not
allowed" due to the hack's insertion of code to define (rather than
just declare) a function in locale.h; so this commit wraps the hack in

Task-number: QTBUG-62878
Change-Id: Ic3fe9280b1b4a84ca569f448351f3a751a79caa5
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>